#include <lib9.h> /* * Return pointer to first occurrence of s2 in s1, * 0 if none */char*strstr(char *s1, char *s2){	char *p;	int f, n; 	f = s2[0];	if(f == 0)		return s1;	n = strlen(s2);	for(p=strchr(s1, f); p; p=strchr(p+1, f))		if(strncmp(p, s2, n) == 0)			return p;	return 0;}